1. Field of the Invention
This invention in general relates to photographic optical systems but in particular to a method and optical apparatus by which a camera initially adapted to utilize a two part zooming optical system can be adapted to utilize only one part of the two part zooming optical system.
2. Description of the Prior Art
Zooming optical systems for use in motion picture cameras are well-known in the photographic arts and usually consist of two separate lens sections, a variable power afocal section whose focal length can be continuously adjusted by movement of one or more groups of its elements and a fixed focus prime lens section which accepts rays emerging from the variable power afocal section to form the final image on the film.
By varying the focal length of the afocal section, a corresponding change in the magnification of the image "seen" by the prime lens is effected while the rays emerging from the variable power section are kept collimated or nearly so with changing magnification.
The basis upon which the focal length of the variable power afocal section can be adjusted resides in the fact that in a lens system which consists of positive and negative lens elements a change in the separation of the elements changes its focal length. This, however, also alters the lens to image distance for maximum image sharpness. Since photographic cameras usually have fixed film planes and it is essential that image sharpness not be changed with changing focal length, such variable focus afocal sections are further structured in such a way that movement of one or more groups of its lens elements is effected so that image sharpness at the film plane is maintained more or less constant with changing focal length or magnification. This is accomplished by what is known as compensation which is achieved in essentially two known ways, mechanically or optically.
In mechanically compensated variable focus sections, movement of the lens element which changes focal length may be linked with the movement of another lens element which compensates for a change in back focus and so keeps the image sharp. These two movements are unequal and are therefore linked by a cam system.
In the optically compensated system, two or more lens components may be fixedly connected together in such a way that they both move the same distance yet keep the image sharp at different focal lengths. Thus, the optically compensated system involves no variable separation between its moving elements.
For a more detailed description of such zooming optical systems, reference may be made to U.S. Pat. No. 3,014,406 issued to Frank G. Back on Dec. 26, 1961 and entitled "Varifocal Lens Assembly For Still Camera Photography"; U.S. Pat. No. 3,890,036 issued to David S. Grey on June 17, 1975 and entitled "Zoom Lens System"; and U.S. Pat. No. Re 29,237 reissued to Trude Muszumanski et al. on May 31, 1977 and entitled "Zoom Lens For Super 8 Film".
In terms of lens performance, zooming optical systems inevitably represents a compromise of greater tolerance than is accepted for a high class lens of fixed focal length. This is because the various lens aberrations, normally corrected for specific inter-element separations, have to be kept constantly low over the whole movement range of the movable elements in the zooming optical system. The compromise generally is achieved by using the aberration characteristics of one or more fixed elements to cancel those of the moving elements. In some zooming optical systems, however, the characteristics of the fixed focus prime lens section elements are designed in such a way that the peformance of the prime lens section by itself is allowed to intentionally degrade in certain respects to cancel aberrations in the variable focus afocal lens section.
Consequently, in zooming optical systems where the aberrations of one section are used to correct those of the other, neither the variable power afocal section nor the fixed focus prime lens section can be used independently for another purpose because neither section when used by itself is favorably corrected for aberrations.
It is therefore a primary object of the present invention to provide a method and optical apparatus by which a camera initially adapted to utilize a zooming optical system of the type which includes two separate sections, a variable focus afocal lens section and a fixed focus prime lens section neither of which by itself is favorably corrected for aberrations, can easily be adapted to utilize only the prime lens section.
Other objects of the invention in part will be obvious and in part will appear hereinafter. The invention accordingly comprises the method and apparatus possessing the construction, combination of elements, arrangement of parts, and steps which are exemplified in the following detailed description.